1. Field of the Invention
The present invention relates to an air compressor of middle or large size which includes an improved suction valve.
2. Prior Art
Referring to FIG. 1 which is a horizontal cross section of a conventional air compressor and FIG. 2 which is a longitudinal cross section of main parts of the conventional air compressor illustrated in FIG. 1.
The conventional air compressor, as described in Japanese Patent Application No. 31615/1989, includes a cylinder 1 on which is provided through a packing 2 a valve seat plate 3 and a cylinder head (not shown). A suction valve plate 4 something like a cantilever (a lead valve) is fitted on a lower surface of the valve seat plate 3. The suction valve plate 4 is of a shape of V-letter with a round bottom, or of a construction having two shanks generally in the form of straight bands integrally connected with each other through a central curved portion, both ends of which are attached to the valve seat plate 3 so that the ends can be arranged on a diametral line of the cylinder 1. The valve seat plate 3 is formed with three suction holes which can be closed and reopened by the suction valve plate 4. A rectangular discharge valve plate 6 is fitted on an upper surface of the valve seat plate with one end thereof being fixed like a cantilever (a lead valve). The discharge valve 6 can close and reopen a discharge hole 7 formed in the valve seat plate. This type of conventional air compressor also has a protrusion 8 which is provided on the curved portion of the suction valve plate 4. The protrusion 8 is engageable with a cavity or concave 9 formed on an inner periphery of top end surface of a side wall of the cylinder. This construction ensures the suction valve plate 4 of reliable actuation.
In a suction stroke, a piston 10 moves from top dead center to bottom dead center and a suction valve is opened, along with which the open air is sucked through the suction holes 5 in the cylinder 1 via a suction chamber of the cylinder head. On the other hand, in a compression stroke, the piston 10 moves from the bottom dead center to the top dead center and the suction valve is closed, and as soon as the air in the cylinder 1 reaches a predetermined pressure, the discharge valve is opened so that the pressurized air can be fed out to an air tank (not shown) from a discharge chamber of the cylinder head through the discharge hole 7 via piping connected to the discharge chamber. A recess 10a is provided on a head of the piston 10 in order to avoid interference of the suction valve plate 4. The piston 10 is provided with a piston ring 11. A piston chamber 12 is defined the piston 10, the side wall of the cylinder 1 and the valve seat plate 3 on the top of the cylinder 1.
The conventional air compressor of the above-described construction is advantageous in many respects; for example, it has a small gap volume and therefore it requires less reexpansion, it has a large cross section of suction gas flow path, which makes it possible to reduce flow speed of the suction gas, resulting in that flow resistance is low and opening speed of the suction valve decreases, and thus seating speed of the suction valve plate 4 can be decreased. The is advantageous from a point of view of the strength of the suction valve seat 4. Further, the characteristic frequency of the suction valve plate 4 can be increased, giving rise to improved response of the suction valve to its opening and closing.
However, the above-described conventional air compressor has a problem that the base portion of the protrusion 8 tends to be damaged and stress concentration tends to occur in the suction valve plate 4 when it is lifted because the lift amount of the suction valve plate 4 is determined by contact of the protrusion 8 of the V-form suction valve plate 4 with the concave 9 in the cylinder.
Furthermore, the portion between the fitting portion to which the suction valve plate 4 is fitted and the protrusion 8 vibrates as being fanned by suction gas because no restriction is posed on the amount of lift of that portion and abrasion occurs between the concave 9 and the protrusion 8. As a result, the base portion of the protrusion 8 tends to be damaged.
Even when the conventional air compressor has an unloader mechanism of a type such that a push rod is actuated to release the suction valve as soon as the discharge pressure reaches a predetermined pressure, the portion between the fitting portion to which the suction valve plate 4 is fitted and the protrusion 8 is fanned by the suction gas and vibrates because there is no restriction on the amount of lift of that portion as described above. In this case too, abrasion occurs between the concave 9 and the protrusion 8, resulting in that the base portion of the protrusion 8 tends to be damaged.